The purpose of this study is to investigate important sources of neonatal behavioral variation and the possible mechanisms underlying them, and to utilize these sources of behavioral variation to determine the role infant behavior plays in determining caretaker behavior. The plan is to identify individual differences in the response systems of newborn infants and then to present all infants to one caretaker in order to verify the hypothesis that from the outset infant behavior determines caretaker response. On Day 3, 60 normal newborn infants will be assessed in regard to their responsivity to tactile stimulation and to an interruption to sucking. Following the assessment, tests of visual habituation will be given the infants and then all infants will be given to one nurse-caretaker for feeding. The feeding interaction will be observed and behaviors of infant and caretaker will be recorded via a multi-keyed event recorder in real time to determine both frequency and duration of infant-caretaker behaviors and to derive the conditional probabilities which obtain the pre-selected behaviors of the caretaker-infant pair. Such an analysis will identify caretaker behavioral variation attributable to individual differences in infant response systems. Observations of infant state will continue for 2 hr after feeding in order to relate infant state measures to infant differences in other response modes. This design allows examination under controlled conditions for the contribution of infant effect determining caretaker-infant interaction. In order to control for variation in caretaker behavior attributable to sex differences, the nurse-caretaker will be informed of the true sex of one-third of the infants; false sex identity will be given on one-third, and no identity on the remaining one-third.